You might want to try a cup rather than a bottle, so he's not expecting EBM.
Formula does take longer to digest than breastmilk - so it is not surprising that he then doesn't want to feed for quite some time.

Maybe your best option is to give him 3/4 of his usual ebm bottle and then try the formula for a few feeds? And if the formula doesn't make him unhappy, just not hungry, then I would be using it last feed for the day, when you want him to take longer before needing feeding again.
Hop over to the exclusive expressers thread too - there are people there who can help you both with cutting back pumps without getting sick yourself, and with this.